Another mum shared this tip with me a while back when I was bemoaning yet another top ruined by 'moon squirter' tomatoes!

All you need to do is wash the item of clothing as normal and then hang it out to dry (stain upwards / outwards) on a bright, sunny day.  At first, you'll think the stain hasn't come out in the wash, but by the time the clothes are dry you'll be amazed.  With any luck there'll be no trace of the tomato stain.  I think the sunlight just bleaches it out or something.  Can't promise it'll work every time, but it's got to be worth a try ... although I suppose it's more of a summer solution than a winter one!